Compared with traditional front-wheel-steering system, the four-wheel-steering system can improve the auto handing stability at high speed as well as the steering response at low speed. Load transfer caused by the body roll can change the tyre cornering characteristics as well as the lateral stability and the bifurcation characteristics of the 4WS vehicles. Therefore, a closed loop system dynamic model considering load transfer was established with the nonlinear coupling relationship between the body and chassis taken into account. Firstly, the existence and stability of the Hopf bifurcation were qualitatively analyzed, then it was compared with a closed loop system dynamic model, the stable region and the characteristics of the Hopf bifurcation were calculated by numerical methods. The results showed that the difference between two dynamic models was obvious. For the 3-DOF system, with the increase of preview distance, steering ratio, road adhesion coefficient and the decrease of the height of suspension roll center, the stable region of the system tended to increase. With the increase of road adhesion coefficient and the steering ratio, the amplitude of self-excited vibration of roll angle and roll angular velocity tended to decrease.